The present invention is generally directed to processes for the preparation of toners, and more specifically to economical processes for modifying toner resin characteristics, and the preparation of toner compositions thereafter. More specifically, the present invention relates to melt mixing processes, batch or continuous, and preferably continuous processes, for example, extrusion for the preparation of toner compositions, and wherein the toner resin is comprised of certain gel or crosslinked fractions generated during resin or toner preparation, reference U.S. Pat. Nos. 5,376,494 and 5,227,460 the disclosures of which are totally incorporated herein by reference, and which applications illustrate, for example, melt mixing processes for preparing toner including a first step comprising a reactive melt mixing process to crosslink an unsaturated base resin and a second step comprising a melt mix process to prepare a toner from the crosslinked resin by incorporating toner additives. With the processes of these copending patent applications, a reactive melt mixing step followed by an additive dispersion step are selected to prepare a toner from a base resin.
In copending patent application U.S. Ser. No. 08/064,773, now U.S. Pat. No. 5,414,052, is disclosed, in embodiments, a one step process for the preparation of toner compositions which comprises adding to an extruder, a base resin, initiator, pigment, and optional charge enhancing additive; effecting crosslinking of the base resin in the extruder to provide a toner comprising a pigment, optional charge additive, and crosslinked resin comprising linear portions and crosslinked portions; and wherein said crosslinked portions are comprised of densely crosslinked gel particles.
Problems associated with the aforementioned processes include the formation of large unpigmented gel particles, as determined by, for example, optical microscopy, during the reactive extrusion of a mixture comprised of a reactive or unsaturated resin and a free radical initiator compound, optionally a pigment, and other optional additives. The unpigmented gel particles residing in pigmented toner compositions are believed to be aggregates of microgel particles, hereinafter referred to as macrogel particles, and are believed to be responsible for producing a print image defect known as mottle or spots and or streaks that possess considerable color variation in a printed image. The aforementioned problem was particularly apparent at low toner mass per unit area (TMA) deposition regions. Another problem of the aforementioned processes concerned the inhibition of free radical reactive extrusion crosslinking processes in the presence of the pigment materials, for example, carbon black, which is a known free radical scavenger. The present invention, in embodiments, provides processes for preparing toner compositions comprising forming a melt mixture with a partially crosslinked toner resin, pigment, a wax, and optional additives in an extruder or equivalent melt mixing device which is suitable for conducting melting and mixing operations to form a predispersed toner mixture; and thereafter conducting at least one additional melt mixing operation on the resulting predispersed toner mixture under temperature or shear conditions that are different from the aforementioned melt mixing to provide a toner with a well defined gel content and an operator controllable melt index.
In other embodiments the present invention is directed to a process for the preparation of pigmented toner compositions comprising: forming at a first temperature, a first melt mixture comprised of a partially crosslinked thermoplastic resin, pigment, at least one wax, and optional additives, wherein the partially crosslinked thermoplastic resin is comprised of a mixture of crosslinked resin macrogel particles, crosslinked resin microgel particles, and uncrosslinked resin; and melt mixing at a second temperature, the first melt mixture to form a second mixture, wherein macrogel particles are partially converted into microgel particles, and wherein the second temperature is less than or equal to the first temperature. In other embodiments, the aforementioned resultant second mixture may be repeatedly melt mixed, for example from 1 to about 10 times, to further reduce the relative ratio of macrogel particles to microgel particles. Additional melt mixing steps of the first pigmented toner mixture provides opportunities for further reducing the particle size of the macrogel particles and which macrogel content and particle size reduction may be used to further enhance the electrophotographic developmental properties of the final toner composition, and the print or copy quality of the imaged toner. Thus, with the processes of the present invention, unpigmented macrogel particles are reduced or substantially eliminated from the resulting toner composition thereby providing toners that are subsequently capable of producing developed electrophotographic images with improved print quality, particularly with respect to color images and transparency uniformity by reducing or eliminating image ghosting or offset and large gel particles deletions.
The toner prepared in accordance with the processes of the present invention can be selected for heat fixable imaging and printing, such as xerographic methods, and wherein there results excellent fusing, vinyl offset performance, and enhanced print quality and uniformity.
Toner utilized in development in the electrographic process is generally known and is prepared by mixing and dispersing a colorant and a charge enhancing additive into a thermoplastic binder resin, followed by micropulverization. As the thermoplastic binder resin, several polymers are known and may be selected including polystyrenes, styrene-acrylic resins, styrene-methacrylic resins, styrene-butadiene resins, polyesters, epoxy resins, acrylics, urethanes and copolymers thereof. As the colorant, carbon black is utilized often, and as the charge enhancing additive, alkyl pyridinium halides, distearyl dimethyl ammonium methyl sulfate, and the like are known.
Toner can be fixed to a support medium such as a sheet of paper or transparency by different fixing methods. A fixing system which is very advantageous in heat transfer efficiency and is especially suited for high speed electrophotographic processes is hot roll fixing. In this method, the support medium carrying a toner image is transported between a heated fuser roll and a pressure roll with the image face contacting the fuser roll. Upon contact with the heated fuser roll, the toner melts and adheres to the support medium forming a fixed image.
Fixing performance of the toner can be characterized as a function of temperature. The lowest temperature at which the toner adheres to the support medium is referred to as the cold offset temperature (COT), and the maximum temperature at which the toner does not adhere to the fuser roll is known as the hot offset temperature (HOT). When the fuser temperature exceeds HOT, some of the molten toner adheres to the fuser roll during fixing and is transferred to subsequent substrates containing developed images, resulting for example in blurred images. This undesirable phenomenon is called offsetting. Between the COT and HOT of the toner is the minimum fix temperature (MFT) which is the minimum temperature at which acceptable adhesion of the toner to the support medium occurs, as determined by, for example, a creasing test. The difference between MFT and HOT is called the fusing latitude.
The hot roll fixing system described above and a number of known toners presently used therein exhibit several problems. First, the binder resins in the toners can require a relatively high temperature in order to be affixed to the support medium. This may result in high power consumption, low fixing speeds, and reduced life of the fuser roll and fuser roll bearings. Second, offsetting can be a problem; third, toner containing vinyl type binder resins such as styrene-acrylic resins may have an additional problem which is known as vinyl offset. Vinyl offset occurs when a sheet of paper or transparency with a fixed toner image comes in contact for a period of time with, for example, a polyvinyl chloride (PVC) surface containing a plasticizer used in making the vinyl material flexible such as, for example, in vinyl binder covers, and the fixed image adheres to the PVC surface.
Thus, there is a need for a toner prepared by simple economical processes which has a low fix temperature and a high offset temperature, or in the alternative, a wide fusing latitude, superior vinyl offset property and superior print quality characteristics. Toners which operate at lower temperatures can reduce the power needed for operation and increase the life of the fuser roll and the high temperature fuser roll bearings. Additionally, such low melt toners, that is, for example, toners having an MFT lower than 200.degree. C., and preferably lower than 160.degree. C., would reduce the volatilization of release oil such as silicone oil which may occur during high temperature heating operation and which can cause problems when the volatilized oil condenses in other areas of the machine. In particular, toners with a wide fusing latitude and with acceptable toner particle elasticity are needed. Toners with wide fusing latitude can provide flexibility in the amount of oil needed as release agent and can minimize copy quality deterioration related to toner offsetting to the fuser roll.
To lower the minimum fix temperature of the binder resin, in some instances the molecular weight of the resin may be lowered. Low molecular weight resins, such as amorphous polyester resins and epoxy resins, have been used for low fixing temperature toners. For example, the use of polyester resins as a toner binder is disclosed in U.S. Pat. No. 3,590,000 to Palermiti et al., and U.S. Pat. No. 3,681,106 to Burns et al. The minimum fixing temperature of polyester binder resins can be lower than that of other materials, such as styrene-acrylic and styrene-methacrylic resins. However, this may lead to a lowering of the hot offset temperature, and as a result decreased offset resistance. In addition, the glass transition temperature of the resin may be decreased, which may cause the undesirable phenomenon of blocking of the toner during storage.
To prevent toner from offsetting to the fuser roll and to increase fusing latitude of toners, various modifications of binder resin structure have been made, for example, by branching or crosslinking. In U.S. Pat. No. 3,681,106 to Burns et al., for example, a polyester resin was improved with respect to offset resistance by nonlinearly modifying the polymer backbone by mixing a trivalent or more polyol or polyacid with the monomer to generate branching during polycondensation. However, an increase in degree of branching may result in an elevation of the minimum fix temperature. Thus, any initial advantage of low temperature fix may be diminished.
Another method of improving offset resistance is to utilize crosslinked resin in the binder resin. For example, U.S. Pat. No. 3,941,898 to Sadamatsu et al., discloses a toner in which a crosslinked vinyl type polymer is used as the binder resin. Similar disclosures for vinyl type resins are made in U.S. Pat. Nos. Re. 31,072 (a reissue of 3,938,992) to Jadwin et al., 4,556,624 to Gruber et al., 4,604,338 to Gruber et al., and 4,824,750 to Mahalek et al.
While significant improvements can be obtained in offset resistance, a major drawback may ensue in that with crosslinked resins prepared by conventional polymerization, that is crosslinking during polymerization using a crosslinking agent, there exist three types of polymer configurations: an uncrosslinked linear and soluble portion referred to as the linear portion; a portion comprising highly crosslinked gel particles or high density crosslink gel particles which is not substantially soluble in organic solvents, like tetrahydrofuran, toluene and the like, and is called gel; and a second crosslinked portion which is low in crosslinking density and therefore is soluble in some solvents, such as, tetrahydrofuran, toluene, and the like, and is referred to as sol. The presence of highly crosslinked gel in the binder resin increases the hot offset temperature, but at the same time the low crosslink density portion or sol increases the minimum fix temperature. An increase in the amount of crosslinking in these types of resins results in an increase not only of the gel content, but also of the amount of sol or soluble crosslinked polymer with a low degree of crosslinking in the mixture. This results in an elevation of the minimum fix temperature, and as a consequence, in a reduction or reduced increase of the fusing latitude. Also, a disadvantage of crosslinked toner resin polymers prepared by conventional polymerization and concurrent crosslinking is that the compatibility of the crosslinked resin with other binder resins may be relatively poor and often exhibit vinyl offset.
Crosslinked polyester binder resins prepared by conventional polycondensation reactions have been provided for improving offset resistance such as, for example, in U.S. Pat. No. 3,681,106 to Burns et al. As with crosslinked vinyl resins, increased crosslinking as obtained in such conventional polycondensation reactions may cause the minimum fix temperature to increase. When crosslinking is accomplished during polycondensation using tri- or polyfunctional monomers as crosslinking agents with the polycondensation monomers, the net effect is that apart from obtaining highly crosslinked high molecular weight gel particles, which are not soluble in substantially any solvent, the molecular weight distribution of the soluble part widens due to the formation of sol or crosslinked polymer with a very low degree of crosslinking, and which sol is soluble in some solvents. These intermediate molecular weight gel species may result in an increase in the melt viscosity of the resin at low and high temperature, which can cause the minimum fix temperature to increase.
Crosslinked polyester binder resins prepared by a reactive melt mixing process have been disclosed in copending application U.S. Pat. No. 5,227,460, the disclosure of which is totally incorporated herein by reference. In this process, the crosslinking reaction is accomplished with a chemical free radical initiator compound when the unsaturated polyester polymer is in the molten state. The resultant partially crosslinked resin mixture comprises crosslinked portions and linear portions. The crosslinked portions comprise very high molecular weight densely crosslinked microgel particles having a volume average diameter of less than about 0.1 micron and are insoluble in substantially any solvent. The linear portion comprises lower molecular weight uncrosslinked resin which is soluble in various common organic solvents. Substantially no portion of the partially crosslinked resin mixture comprises sol or polymer with low degree of crosslinking. The crosslinked portions or microgel particles are prepared in such a manner that there is substantially no distance between the polymer chains, that is the crosslinked polymer chains are joined together by a single covalent bond and without intervening chemical structure. This crosslink structure is different from conventional crosslinking in which the crosslink distance between chains is quite large with several or more monomer units providing intervening chemical structure. The highly crosslinked microgel particles are distributed throughout the linear portion and impart elasticity to the resin, and which elasticity improves the resin offset properties, while not substantially affecting the resin minimum fix temperature. The aforementioned melt mixing process disclosed in U.S. Pat. No. 5,227,460 is a reactive melt mixing process whereby a base resin, for example an unsaturated polyester, is converted into a resin mixture comprised of an uncrosslinked linear fraction and a crosslinked fraction.
Many processes are known for effecting polymerization reactions, including reactive melt mixing processes, for both initial polymerization reactions employing monomers or prepolymers, and for polymer modification reactions, such as grafting, coupling, crosslinking, and degradation reactions. The process is generally known as a reactive extrusion process when the melt mixing device is an extruder. The reactive extrusion process is particularly advantageous for polymer modifications in many respects. The modification generally takes place when the polymer is in molten state, thus eliminating the use of large amounts of solvent whose handling is both difficult and costly. The extrusion process is inherently easier to control as compared to a large polymerization reactor vessel.
In the aforementioned U.S. Pat. Nos. 5,376,494 and 5,227,460 are disclosed processes whereby polymers are crosslinked using a chemical free radical initiator as a crosslinking promoting agent in the molten state at high temperature in an extruder. The partially crosslinked resin mixture prepared by the reactive extrusion process is subsequently melt blended again with a colorant, charge enhancing additives and the like, and result in a toner mixture prior to pulverizing operation to obtain toner particles. Although the first reactive extrusion operation can prepare a toner resin comprising very high molecular weight densely crosslinked microgel particles which improve the resin offset properties, the need to subject the resin to a second extrusion operation wherein intensive mixing and heating is employed to disperse toner additives and colorant can lead to the formation of macrogel particles which are frequently unpigmented. The unpigmented macrogel particles can produce the aforementioned print defects.
U.S. Pat. No. 5,057,392 to McCabe et al. discloses a low fusing temperature toner powder which employs a polyblend of a crystalline polyester and an amorphous polyester that has been crosslinked with an epoxy novolac resin in the presence of a crosslinking catalyst. The mixture, which includes the polyesters, the epoxy novolac resin, catalyst and colorant, is melt blended on heated compounding rolls or by passage through an extruder. During melt blending, the amorphous polymer is crosslinked with the epoxy novolac resin. Crosslinking substantially increases the offset latitude of the mixture. After melt blending, the mixture is annealed by being maintained at a temperature above the glass transition temperature of the amorphous polyester, but below the melting temperature of crystalline polyester, preferably in the range of 50.degree. to 80.degree. C. The annealing is continued for a time sufficient for the crystalline polyester to recrystallize as dispersed small particles within a matrix phase comprised of a crosslinked polymeric reaction product of the amorphous polyester and the epoxy novolac resin. Typical annealing times are in the range of about 0.2 to about 2 hours. If annealing is not carried out, the polyblend does not have the desired grindability and the toner powder derived therefrom does not have desired fusing temperature and shelf life or keep characteristic. The melt blending and reaction process is not sufficient to provide a toner with desired properties. An additional annealing step, following melt blending, to recrystallize the crystalline polyester is therefore necessary in order to provide the toner with optimum morphology. Another potential problem not addressed in the patent is the possibility of interference from some active toner additives during crosslinking. For instance, it is known that some carbon black pigments will inhibit certain types of polymer reactions.
Therefore, there remains a need for a toner composition and fabrication processes during which the toner properties can be optimized and controlled by redistribution of gel phase content from macrogel to microgel with, for example, high shear mixing, in a simple straightforward manner and thereafter provide toner particles which are substantially free of unpigmented macrogel particles.